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/******************************************************************************
 * Copyright (c) 2004, 2005 IBM Corporation and others.
 * This program and the accompanying materials are made
 * available under the terms of the Eclipse Public License 2.0
 * which is available at https://www.eclipse.org/legal/epl-2.0/
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 * SPDX-License-Identifier: EPL-2.0

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 * Contributors:
 *    IBM Corporation - initial API and implementation 
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Provides the base interfaces for all service providers. The following examples are some of the services in the modeling platform:
<ul>
<li>Layout Service
<li>Property Service
<li>Decorator Service
</ul>
<p>
The modeling platform consists of a set of services whose requests are fulfilled by service providers that implement {@link org.eclipse.gmf.runtime.common.core.service.IProvider IProvider}.
Each service exposes an extension-point that providers extend to be discovered and used. This package contains classes that define the abstract contract for service providers.</p>
</p>

<h2>Package Specification</h2>
<p>
When it receives a client request, a service typically embeds the request information in an {@link org.eclipse.gmf.runtime.common.core.service.IOperation IOperation} and then evaluates the 
extensions of its defined extension-point to find potential providers for the request. To determine to which providers to delegate a request,
a service uses two sequential filters:
<ol>
<li>The service evaluates the information in each provider's extension descriptor. The service extension-point schema typically filters 
out most providers in this step and prevents the unnecessary loading of a provider. 
<li>The service requests information from each provider about whether it can support the request. The provider's
{@link org.eclipse.gmf.runtime.common.core.service.IProvider#provides IProvider.provides} implementation performs this action.
</ol>
</p>
<p>
Typically, the service sorts the providers that passed the filters according to the priority that the service provider extension descriptor defines. 
Depending on the service and the request, the service might request one or more providers to perform the operation. After the required providers are defined, the service
delegates the execution of the request for each provider to the operation's {@link org.eclipse.gmf.runtime.common.core.service.IOperation#execute IOperation.execute} implementation.
Each service provides its own IOperation subclass that subsequently delegates to the provider.
</p>
<p>
Most services expose an abstract class implementing IProvider that providers should subclass. The abstract class specifies the contract between the service and its providers since 
it specifies the API to perform requests. The abstract class usually extends 
{@link org.eclipse.gmf.runtime.common.core.service.AbstractProvider AbstractProvider} to leverage the notification support of this class.
</p>
<p>
 Some services require notification from a provider if the provider's state changes. This notification occurs through the
 {@link org.eclipse.gmf.runtime.common.core.service.IProviderChangeListener IProviderChangeListener} interface. These types of service connect to their provider using
 {@link org.eclipse.gmf.runtime.common.core.service.IProvider#addProviderChangeListener addProviderChangeListener} and disconnect using
 {@link org.eclipse.gmf.runtime.common.core.service.IProvider#removeProviderChangeListener removeProviderChangeListener}. Services often
 provide their own derived {@link org.eclipse.gmf.runtime.common.core.service.ProviderChangeEvent ProviderChangeEvent} classes to convey more
 service-specific information about the state change.
</p>
<p>
Sometimes, it is impossible to provide enough information in a service provider extension descriptor to avoid
the unnecessary loading of the provider plug-in. When the cost of loading a provider plug-in is high, it might be preferable to move
the implementation of {@link org.eclipse.gmf.runtime.common.core.service.IProvider#provides IProvider.provides} to a lightweight plug-in. 
To do this, a service provider extension descriptor should include a "Policy" subelement with
a class attribute that references a class that implements {@link org.eclipse.gmf.runtime.common.core.service.IProviderPolicy IProviderPolicy}. This
interface is a subset of IProvider that only includes the
{@link org.eclipse.gmf.runtime.common.core.service.IProviderPolicy#provides IProviderPolicy.provides} member.
</p>
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